Embodiments of the present invention relate to circuits in general and, in particular, to output buffer circuits.
In electrical circuits, an output buffer may be used to isolate an input signal from an output signal. This may prevent the input signal from adversely affecting other circuits that the output signal interacts with. Further, the input signal may be weak. An output buffer may provide a stronger output signal, capable of driving a capacitive load. In some situations, the maximum output voltage of an output buffer is equal to the input voltage less the threshold voltage of the electrical device used to create the buffer. For example, such a situation may arise when the output buffer is a CMOS buffer, with the pull-up device of the driver circuit being an NMOS transistor. In such situations, the maximum output voltage is lower than the input voltage. In some situations, an increased output voltage may be desirable. For example, if the maximum output voltage is reduced, there may not be enough of a voltage swing—the difference between the high output voltage and the low output voltage—to properly interface with another circuit.
In some low power applications, a 0.8V output swing from a differential clock circuit may be desired. However, a traditional output buffer architecture may not create a large enough voltage swing for a functional differential clock signal when a lower voltage supply is used. For example, a nominal 1.5V power supply may have an output voltage as low as 1.4V. Such a situation may leave only 0.5V for the differential output swing.
Since the use of a low voltage supply may be desirable, a new design of output buffers is described that may allow for a low voltage supply to power the device, yet may create an output swing voltage greater than is possible using a standard circuit architecture.